Properties of (Bi_1/9Na_2/3)(Mn_1/3Nb_2/3)O₃ analysed within dielectric permittivity, conductivity, electric modulus and derivative techniques approach

Abstract

The (Bi_1/9Na_2/3)(Mn_1/3Nb_2/3)O₃ ceramics with perovskite structure were sintered. The XRD test proved that the samples are cubic (a = 3.920 ± 0.001 Å). Microstructure and atomic composition were determined with a SEM (JSM-5410) equipped with energy dispersion X-ray analyser (ISIS-300). The fluctuation in the chemical composition was found indicating on local disorder. Broadband dielectric spectroscopy in the range 10⁻¹–3 · 10⁷ Hz was applied within the range of 100–650 K. The real, ϵ′(f, T), and imaginary, ϵ″(f, T), parts of complex dielectric permitivity characteristics, both in the temperature and frequency domain, show relaxation processes partially covered by electric conductivity. At high temperatures the electric conductivity exhibits a thermally activated behaviour σ(f, T) ∝ exp(−E _a/kT) but the variable range hopping (VRH) dependence σ ∝ exp[−(T ₀/T)^1/4] is manifested at low temperatures. The derivatives technique in the frequency (∂ log ϵ/∂ log ω) and temperature (∂ log ϵ/∂T) domain enabled various relaxation processes to be distinguished. The data converted to electric modulus representation, M*(f, T) = 1/ϵ*, exhibited clearly resolved relaxation peaks. The relaxation times obtained from the peaks position show a slightly non-Arrhenius temperature behaviour with the activation energy varying in 0.4–0.6 eV range and characteristic time of the electric conductivity relaxation of the order of 10⁻¹² s. The relaxation times can be fitted at better accuracy with the VRH dependence where T ₀ are of the order of 10⁸ K. It is shown that the low frequency ac-conductivity converges to dc-conductivity and the relation σ(0) ∼ ω_m ∼ τ_m ⁻¹ typical for the disordered solids applies. The conduction current relaxation relationship behaves in accord with the VRH system: σ_dc ∝ (T/T ₀)^q (e ²/kT) ω_c, where ω_c = ν_ph exp[−(T ₀/T)^1/4] is valid for the locally disordered (Bi_1/9Na_2/3)(Mn_1/3Nb_2/3)O₃ compound.

Keywords:

• Dielectric relaxation
• Electric conduction
• Perovskites
• Variable range hopping